Various precision measuring gauges for sensing linear, rotary or angular displacement or dimensions are currently available. These gauges are generally based on either optical systems, magnetic scales, inductive transducers, or capacitive transducers.
For gauges based on optical systems, a number of different optical displacement measuring systems have been developed. Typically, for gauges that provide high resolution measurements over a comparatively long measuring range, a measuring signal arising from the displacement of an internal grating scale is detected. In such optical displacement measuring systems, the grating scale is illuminated and a set of photodetectors or a photodetector array is arranged to derive four periodic quadrature-type signals from light fringes that arise from grating scale. However, such optically-based measuring gauges have heretofore had several undesirable limitations. One limitation is that such gauges have reached a typical minimum size that makes their use inconvenient or impossible in a number of applications. Another limitation is that such gauges suffer limitations in converting the high frequency photodetector signals associated with high speed gauge displacements and transmitting those signals over long cables without significant signal loss or interference.
Another limitation is that such gauges are typically “incremental” type gauges, that is, the measuring signals arising from the various periods of the periodic grating scale are indistinguishable from one another. Therefore, for displacements exceeding one period, each increment or period of the scale must be accumulated in order to determine the net displacement of the gauge. Absolute type optical gauges are known. However, such gauges tend to be even larger in size than the aforementioned incremental type gauges. Also, absolute type gauges tend to have even lower measurement cycle rates, and thus also suffer limitations in providing the high frequency photodetector signals needed to track high speed gauge displacements in real time.
Various optical encoder systems utilizing optical fibers are known, such as that disclosed in U.S. Pat. No. 4,733,071, to Tokunaga. The system described in the '071 patent has a code member scale, and an optical sensor head comprising an optical fiber tip light emitter and two optical fiber tip receptors closely arranged along the code member measuring axis. However, the accuracy of the resulting encoders have either been relatively crude, or their size has been excessive, or both. Thus, such systems have not been effective for use in precision measuring gauges.
Precision measuring gauges based on magnetic, inductive, or capacitive transducers, in addition to typically providing cruder levels of resolution and accuracy, generally also suffer from the other limitations outlined above.